“It really was just an age thing,” a source close to Katie Lee Joel said yesterday of her separation. “She loves the city, she loves to go out and have fun and he prefers to stay at home.”

When Joel — who at 60 is just shy of being able to collect Social Security — isn’t on the road touring, he likes to tinker with his antique motorcycle collection at home on Long Island and sail on his yachts.

“The age difference didn’t seem to be a big deal seven years ago [when they met], but it has become one as they got older,” the source said. “They just grew apart.”

With the relationship on the rocks, there appears to be little chance that Katie Lee will make off with any significant portion of Joel’s massive fortune. Sources close to him said they had an ironclad prenuptial agreement.

Sources on both sides believed Katie Lee would likely end up with the couple’s Greenwich Village townhouse, which Joel bought for $5.9 million in 2006.

The pair announced their divorce Wednesday amid reports that Joel had grown furious at the amount of time his much younger wife was spending with Israeli designer Yigal Azrouel.

Her presence with the fashionista in Manhattan had raised eyebrows all over town.

But a source close to Katie Lee said yesterday: “It had nothing to do with it [the separation]. There wasn’t some dramatic thing that happened. There was no blow-up or fight.”

Joel — who recently took his $32.5 million Centre Island estate off the market — had a real-estate broker stop by yesterday at the Sagaponack spread he bought for Katie Lee for $16.75 million in 2007. Sources said he wasn’t planning to sell it.

“The house in Centre Island was on the market but then he decided he wanted to keep it,” a source close to him said.

Joel clearly learned a lesson about protecting his money following his divorce from his first wife, Elizabeth Weber, in 1982. He kept her as his business manager and later accused her brother, Frank Weber, of embezzling $90 million.

Joel has sold more than 100 million records worldwide — including 1985’s “Greatest Hits Vols. 1 & 2,” which sold 21 million copies, making it the sixth best-selling record of all time. It ranks behind only AC/DC’s “Back in Black,” Pink Floyd’s “The Wall,” “Led Zeppelin IV,” Michael Jackson’s “Thriller” and The Eagles’ “Greatest Hits 1971-1975.”

He managed to rebuild his fortune after marrying supermodel Christie Brinkley and continues to rake in millions when he tours — charging up to $185 a ticket and packing stadiums.